1. Field of the Invention
This invention relates to a radiation image converting apparatus which employs an accelerated phosphorescence fluorescent screen having a certain rigidity or a low flexibility and more particularly, to a radiation image converting apparatus which has a structure which decreases diffused radiation upon photographing.
2. Description of the Related Art
In recent years, systems wherein a radiation image is recorded on an X-ray film or the like and the film on which a radiation image is recorded is used for observation, diagnosis, etc., have been and are gradually being replaced by another type of system wherein radiation having passed through an object for photographing are irradiated upon an accelerated phosphorescence fluorescent material formed in a sheet or a panel to accumulate and record a radiation image on the accelerated phosphorescence fluorescent material and then the radiation image is photoelectrically read to obtain an image signal, for which suitable image processing is subsequently performed to obtain a reproduction image. Basic construction of a system which employs an accelerated phosphorescence fluorescent material is disclosed in detail in U.S. Pat. No. 3,859,527. Here, the accelerated phosphorescence fluorescent material signifies a fluorescent material which accumulates energy. When radiation such as X-rays, .alpha. rays, .beta. rays or .gamma. rays are irradiated thereupon, and if excitation light such as infrared rays, visible rays or ultraviolet rays is irradiated thereupon within a given period of time, then the accumulated energy is radiated as accelerated phosphorescence light. A kind of radiation energy of which it is easy to accumulate, a wavelength of excitation light with which accelerated phosphorescence light is liable to be radiated, a wavelength of accelerated phosphorescence light to be emitted and so forth are different depending upon different kinds of fluorescent materials.
It is recognized, with a system which employs an accelerated phosphorescence fluorescent material, that the amount of accelerated phosphorescence light emitted from the accelerated phosphorescence fluorescent material by irradiation of excitation light increases in proportion, over a wide range of energy, to the energy of the radiation irradiated upon the accelerated phosphorescence fluorescent material. Further, the ratio between the amount of light and the energy can be varied by an amount of excitation light. Accordingly, a radiation image which is not influenced by a variation in the amount of exposure radiation can be obtained. Further, in a system for obtaining an X-ray image of a human body, the exposure dose to a human body upon X-ray photographing can be reduced.
A radiation image converting apparatus may include a photographing and recording section, including an accelerated phosphorescence fluorescent plate, wherein an accelerated phosphorescence fluorescent material is formed in a panel and is rigid or has limited flexibility, and a reading unit for irradiating excitation light upon the accelerated phosphorescence fluorescent plate which has a radiation image accumulated and recorded thereon, the reading unit receiving accelerated phosphorescence light radiated from the accelerated phosphorescence fluorescent plate to obtain an image signal. The radiation image converting apparatus has a problem such that, when radiation such as X-rays are irradiated upon the accelerated phosphorescence fluorescent plate, part of the irradiated radiation passes through the accelerated phosphorescence fluorescent plate and are scattered in the inside of the apparatus so that they impact upon the accelerated phosphorescence fluorescent plate again, and consequently, the sharpness of a radiation image accumulated and recorded on the accelerated phosphorescence fluorescent plate is deteriorated, so that a visible image reproduced in accordance with an image signal obtained by reading the radiation image is made unclear.
One of the possible solutions to the problem may be to provide, similarly as in a conventional apparatus which employs an X-ray film, a rigid lead plate on the rear face side of the accelerated phosphorescence fluorescent plate such that radiations having passed through the accelerated phosphorescence fluorescent plate may be absorbed by the lead plate, so that they may not be admitted into the inside of the apparatus. In the case of a conventional apparatus which employs an X-ray film, there is no problem if a lead plate is disposed in such a manner as described just above because an X-ray film after completion of photographing is taken out to effect development thereof. However, when an accelerated phosphorescence fluorescent plate is employed, it is necessary to dispose, after completion of photographing, the accelerated phosphorescence fluorescent plate and the reading unit close to each other to effect reading of a radiation image accumulated and recorded on the accelerated phosphorescence fluorescent plate. Therefore, if a lead plate is disposed on the rear face side of the accelerated phosphorescence fluorescent plate, then it is difficult to place, when reading is to be performed, the accelerated phosphorescence fluorescent plate and the reading unit close to each other, which is a serious obstacle to miniaturization of the apparatus and simplification in structure of the apparatus.
Further, while the object is fundamentally different from that of the present invention, a technique that, in an apparatus which includes a combination of a so-called film screen method wherein an X-ray image is photographed and recorded on an X-ray film combined with an intensifying screen and another so-called fluoroscopic photographing method wherein X-rays are converted into an electric signal using an X-ray fluoroscopic intensifying tube and then the X-ray image is observed on a CRT display, a thin lead plate is adhered to the intensifying screen and the intensifying screen is moved to and from a location in front of the X-ray fluoroscopic intensifying tube, is already known and disclosed in Japanese Patent Laid-Open Application No. 2-100042.